The present invention relates to a mold for the servo-driven type mold clamping unit of injection molding machine and, more specifically, to a method of protecting the mold of the servo-driven type mold clamping unit of an injection molding machine.
A regular injection-molding machine is generally comprised of a machine base, a mold-clamping unit, an injection unit, and an electric control system. The mode-clamping unit comprises a mold, a plurality of tie rods, and a toggle mechanism. The mold is comprised of a fixed die and a movable die. The tie rods connect the fixed die and the movable die. The toggle mechanism is connected between a drive member, which generates a driving force, and the movable die, and the toggle member is driven by the drive member to move the movable die back and forth along the tie rods relative to the fixed die.
In order to protect the mold, preventing the movable die from squeezing a foreign object during its closing stroke, a mold protection method is provided. A known mold protection method is to set a mold protection zone and a time limit in which the movable die passes from the start of the mold protection zone to the end thereof during movable die feed stroke, and to let the movable die pass the set mold protection zone at a low pressure and low speed. If the movable die does not pass through the mold protection zone within the set time limit, it is judged that a foreign object exists in the mold and, the movable die is immediately stopped or returned to its initial status.
According to the aforesaid mold protection method, the movable die passes through the mold protection zone at a low pressure and low speed. However, because the movable die is linked to the toggle mechanism that changes its mechanical advantage subject to the change of position, the mechanical advantage of the toggle mechanism applied to the movable die is high and the low pressure driving force is not maintained unchanged when the movable die touches a foreign object, thereby causing a high squeezing force to be produced at the movable die against the existing foreign object. In this case, the mold tends to be damaged. Furthermore, the movable die can be returned to its original position only after he set mold protection time limit is up and the movable die does not pass the end of the mold protection zone. This mold protection method cannot proceed to the abnormality handling procedure (return the movable die to the initial position) immediately upon the presence of a foreign object, resulting in a risk of damage to the mold.
There is known another method to detect the presence of a foreign object in the mold subject to the variation of the current output of the motor. According to this method, a current detection point and a current output point must be set in the controller and the motor respectively. This method cannot eliminate the problems of low resolution and high interference of noises. In actual practice, this method has the disadvantages of low reliability and high cost.
It is the main object of the present invention to provide a servo-driven type mold clamping unit mold protection method, which detects the presence of a foreign object in the mold in real time and, takes the necessary measure immediately upon the detection of the presence of a foreign object in the mold to protect the mold.
To achieve the object of the present invention, the method for protecting a mold of a servo-driven type mold clamping unit comprises the steps of (a) changing the torque output of the power source generated by a servo-motor subject to the position of a movable die of the mold, and (b) comparing the values set in an encoder of the servo-motor with signal received by the encoder when the movable die is in displacement so as to detect the presence of any foreign objects in the mold for determination of normal/abnormal handling control.